The petroleum refining industry generates large quantities of low value process gases which typically have high concentrations of sulfur compounds. These refinery off gas (ROG) streams, as they are known, are generated from various “secondary” processing technologies used in oil refining such as catalytic cracking, hydro-treating and delayed coking processes. The largest quantity of ROG streams are derived from petroleum cracking units.
ROG streams are comprised of a wide range of gases including hydrogen, carbon monoxide, carbon dioxide and hydrocarbons with more than one carbon atoms including both saturated (paraffins) and unsaturated (olefins) hydrocarbons, such as ethane and ethylene respectively. The content of ethane and ethylene can be as high as 30% and the content of hydrogen is typically in the range of 15 to 50%. The sulfur compounds are typically hydrogen sulfide (H2S), carbonyl sulfide (COS) and organic sulfur compounds such as mercaptans, thiophenes and sulfides. The concentration of H2S can be greater than 1% by volume and the concentration of organic sulfur compounds can be several hundred parts per million.
Due to the lack of effective technologies for converting ROG streams into more valuable products or useful feed streams, many of these gas streams are used for their fuel value or, in many cases, simply flared. However, even the simple combustion of ROG streams containing high concentrations of sulfur compounds can result in the emission of toxic or other environmentally undesirable gases such as sulfur oxide compounds. Stringent environmental regulations for the emission of these undesirable compounds require that refineries invest in expensive scrubbing systems for more complete sulfur removal from ROG streams prior to or after combustion.
The conversion of high sulfur ROG streams into more valuable low sulfur, hydrocarbon/hydrogen containing streams can reduce energy losses, provide valuable feed streams for further processing, and eliminate many of the environmental concerns associated with the combustion of high sulfur ROG streams. Moreover, since many hydrocarbon conversion processes are catalytic using expensive metal catalysts, the sulfur concentration must be lowered to avoid poisoning the metal catalysts in order to effectively use the hydrocarbon/hydrogen content in the ROG streams as feed gases.
Generally, ROG streams are taken from multiple refinery processing units, collected and desulfurized at a central location in the refinery. However, ROG streams may be required to be taken from a single refinery process and treated and/or used without mixing with other off gas streams due to its specific gas composition.
Many refineries already use amine sulfur removal technology. Amine sulfur removal technology is well known and refers to a group of processes that use aqueous solutions of various amine compounds (commonly referred to simply as amines) to remove H2S and carbon dioxide (CO2) from sulfur containing gases. While these amine systems are very effective at removing H2S, they are less effective in removing organic sulfur species such as mercaptans, thiophenes, sulfides, and other complex sulfur compounds. For the removal of these organic sulfur compounds, the use of caustic removal systems is generally needed. Caustic removal systems are expensive, use caustic reagents such as potassium hydroxides, which are considered toxic, become consumed and require safe environmental disposal.
Another desulfurization option for fuel gas streams containing organic sulfur compounds is a two-step process consisting of hydrodesulfurization, e.g. the conversion of organic sulfur compounds to H2S, and the subsequent removal of the H2S with an amine based system or a solid sulfur adsorbent such as ZnO. This approach is typically used for the desulfurization of natural gas feedstocks and ROG streams having low sulfur levels (e.g. 5-10 ppm) such as when natural gas is used as a feedstock in a steam methane reformer for the production of hydrogen. Conventional hydrotreaters in steam methane reformer based hydrogen plants operate at about 300° C.-400° C. utilizing waste heat from the steam methane reforming plant to preheat the feed to the hydrotreater. The catalyst used in conventional hydrotreaters is typically a CoMo or NiMo catalyst.
As mentioned above, the organic sulfur compounds in the ROG streams can be first hydrogenated in a hydrotreating process to form H2S and then subsequently removed with conventional amine sulfur removal systems. However, for efficient hydrotreating of organic sulfur compounds, heat must be supplied and removed both economically and reliably for the system to convert organic sulfur to H2S. Since the ROG streams are typically received at low pressures, such as 5-10 bar, the hydrotreater must be operated at elevated temperatures in the range of 290°-370° C. to ensure complete conversion of the organic sulfur species. Controlling the temperature within the hydrotreater becomes a key to finding a cost effective sulfur removal process because waste heat is not always available.
Achieving the high temperature needed for hydrogenation of organic sulfur compounds without using an external heat source can be a problem. Hydrogenation of gas streams containing olefins is an exothermic reaction thereby providing heat to the reaction. If the ROG stream does not contain sufficient concentration of olefins, the hydrogenation system will not be able to maintain the proper temperature for conversion of the organic sulfur compounds to H2S and external heat must be provided to the reactor. If the ROG stream contains too high of a concentration of olefins, the hydrotreating unit can overheat causing the catalyst to be damaged or destroyed.
One solution to this problem is to dilute the high olefin containing ROG stream with a recycle stream from the hydrotreater product. This however requires a recycle compressor which complicates the system, makes it less reliable and increases the cost. Also, ROG streams usually have significant composition variability which makes a hydrotreater with recycle compressor based system difficult to design and control. Last, due to the typical low pressure of ROG feed streams, the hydrotreater operates at low space velocities, such as less than 1000 hr−1, which require that the reactors be extremely large adding additional capital costs. Space velocity is defined here as the volumetric flow of ROG streams at standard conditions (standard m3/hr) divided by the reactor volume (m3). Since the cost of the catalytic reactor catalyst is significantly higher than the cost of the conventional hydrotreater catalyst, the better solution could be a combination of the two reactors depending on the management of operating factors such as catalyst cost, pressure and olefin concentration.
Thus the present invention provides a sulfur processing system that is flexible enough to process ROG streams having varying sulfur concentrations, varying organic sulfur compounds, and varying olefin content while still being economical. This invention uses no continuously supplied external heat source for the hydrogenation reaction, eliminates the need to use recycle streams to the hydrotreater to control temperature, and allows for the use of smaller reactors reducing capital costs. Last, the present invention allows for the elimination of a caustic sulfur removal systems and replaces them with a process employing a catalytic reactor used with an amine absorber that is more reliable, easier to operate and can be integrated with the existing refinery amine system.